1. Field of the Invention
The present invention relates to a method for coating a steel strip and an apparatus therefor.
2. Description of the Related Arts
FIG. 5 shows a conventional apparatus for continuous hot-dip coating. The method for hot-dip coating using the apparatus is described below.
A steel strip S is continuously annealed in an annealing furnace while cleaning the surface thereof at a time, then the steel strip S is passed through a coating pot 4 to apply coating thereto. Since the annealing step is normally conducted in a reducing atmosphere, a snout 3 which has a rectangular cross section is located between the annealing furnace and the coating pot 4 to keep the reducing atmosphere. Thus the steel strip S passes through the snout 3 and enters into the coating pot 4 which contains a molten metal to perform the specified metal coating without exposing the steel strip to environmental air. The running direction of the coated steel strip S is changed by a sink roll 6 in the coating pot 4 to rise vertically to be taken out from the coating pot 4. The coating thickness of the steel strip S which was taken out from the coating pot 4 is adjusted to a predetermined level using a gas-wiping nozzle 7, then the steel strip S is cooled by a cooling unit (not shown), further is sent to a succeeding step for undergoing temper rolling or other treatment, at need.
The atmospheric gas is supplied into the furnace through a cooling zone 1 at the exit side of the annealing furnace or through the snout 3, and flows toward the inlet side of the annealing furnace, which flowing direction is opposite to the running direction of the steel strip S. Since the inside of the snout is kept in a reducing atmosphere, an oxide film is hardly formed on the surface L of, the molten metal bath in the snout. As a result, the molten metal is exposed directly onto the bath surface, which results in the evaporation of the molten metal to a saturated vapor pressure at the temperature of the molten metal bath. The vapor of evaporated molten metal reacts with a slight amount of oxygen which exists in the reducing atmosphere within the snout and within the annealing furnace to yield an oxide.
Even when the metal vapor is not converted to an oxide, if the vapor pressure of the evaporated molten metal exceeds the saturated vapor pressure thereof in the evaporated zone, the evaporated molten metal returns to the metallic state because the evaporated molten metal cannot sustain its vapor phase. Particularly when the temperature at the cooling zone in the annealing furnace and at the internal face of the snout is at or below the saturation temperature at the vapor pressure of the evaporated molten metal, the metal vapor condenses to become metal powder, which metal powder then deposits onto the inner face of the furnace and of the snout.
If the oxide and deposit directly attach to the cleaned steel strip during the treatment, irregular coating or absence of coating may appear, which induces quality defects caused by dross deposition.
If the oxide drops onto the surface L of the molten metal bath in the snout, the oxide does not dissolve in the molten metal bath M because the melting temperature of the oxide is higher than the temperature of the molten metal. When the deposit drops onto the surface L of the molten metal bath in the snout, the deposit is remelted if it is the same metal with the molten metal. In most cases, however, the deposit contains impurities so that it does not dissolve in the molten metal bath M.
The oxide and deposit which do not dissolve after dropping onto the molten metal float on the surface L of the molten metal bath in the snout, then flow along the stream of the molten metal bath M accompanied with the steel strip entering the coating bath, thus migrate toward the steel strip and finally attach thereto. In that case, also, the deposit acts as an interference cause against a coating action, so the coating thickness becomes thin and an irregular coating appears, which induces quality defects caused by dross deposition.
Various methods have been introduced to prevent the generation of quality defects caused by dross deposition in the snout. These proposed methods are roughly classified into two groups.
The first group is a method to remove impurities which dropped onto the surface of bath in the snout to outside of the snout. For example, JP-A-2-70049 (the term "JP-A" referred to herein signifies "unexamined Japanese patent publication"), JP-A-4-120258, and JPA-5-279827, (hereinafter these patent publications are referred to simply as "the Prior Art 1") disclose a method to prevent the occurrence of quality defects caused by dross deposition by continuously guiding the molten metal from inside of the snout to outside thereof, thus removing impurities dropped in the snout and simultaneously maintaining fresh surface of the molten metal bath. According to the Prior Art 1, a pump is installed either in or above the molten metal bath to induce the molten metal flow.
The second group is a method to reduce the occurrence of quality defects by suppressing the generation of oxide in the snout. For example, JP-A-6-49610, (hereinafter the patent publication is referred to simply as "the Prior Art 2"), discloses a method to suppress the generation of dross on the surface of molten metal bath in the snout by locating a seal at an upper portion of the snout while contacting or non-contacting the steel strip, and by injecting a gas having a stronger reducing performance than that of the reducing atmosphere in the annealing furnace into the snout between the seal and the molten metal bath.
Prior Art 1 uses a pump for transferring the molten metal. For the case that the molten metal is molten zinc, for instance, the molten zinc severely erodes other metals so that the life of the pump is significantly short, or about 3 months at the longest. Therefore, Prior Art 1 has a problem of durability of facilities. Furthermore, Prior Art 1 does not remove metal vapor from the system. Thus Prior Art 1 provides no full scale problem solving.
According to the method described in Prior Art 2, the surface of the molten metal bath is cleaned to reduce the oxide film formation. As a result, evaporation of metal from the surface of molten metal bath is further enhanced. The reducing gas containing evaporated metal passes through the seal in the snout, flows from the snout into the annealing furnace, then condenses in the snout and in the annealing furnace, or reacts with a slight amount of oxygen in the furnace to become an oxide, which forms deposit in the snout and the annealing furnace. As described above, that type of deposit directly adheres to the steel strip, or floats on the surface of molten metal bath in the snout, and accumulates with operation time to induce quality defects caused by dross deposition. Therefore, Prior Art 2 needs to have an additional means to solve the surface defect problem. Consequently, Prior Art 2 is insufficient as a preventive method against quality defects caused by dross deposition.
That is, there has not been developed a molten metal coating method that has a strong effect of preventing quality defects caused by dross deposition in the snout, or an apparatus therefor having excellent durability.